Conventionally, detection and ranging systems that detect a target using radio waves (e.g., millimeter waves) have been put into practical use. The detection and ranging systems radiate a probe signal to a detection region and receive a reflected signal (i.e., an echo signal) from a target. By analyzing the received echo signal, the detection and ranging systems estimate a line-of-sight direction relative distance to the target (hereinafter simply referred to as “distance”), a line-of-sight direction relative speed of the target (hereinafter simply referred to as “speed”), and the direction to (or angle formed with) the target. Such detection and ranging systems include, for example, a plurality of receiving sensor elements arranged at equal spacing and detect a target according to, for example, the phase of an echo signal received by each receiving sensor element. That is, for a target within a detection region, factors (such as distance, speed, and angle) are estimated. As an example, Japanese Laid-open Patent Publication No. 2010-145289 is known as one related art.
Some detection and ranging systems that detect a target using a transmitting sensor array having M sensor elements and a receiving sensor array having N sensor elements generate a signal vector based on an echo signal from the target and perform signal processing on the signal vector so as to estimate the direction of arrival of the echo signal (i.e., the angle formed with the target). Such systems and, in particular, detection and ranging systems including radars use a technology called “spatial smoothing” in order to eliminate coherency of received signals.
The echo signal from each target includes angle information of the target. An angle matrix describes, as phase information, the position of a sensor element that receives the signal.
However, in a case in which a plurality of angle vectors that form the angle matrix include angle vectors that are complex-conjugate to each other (this case may more generally be represented as a case in which a certain angle vector is complex-conjugate to a vector represented by a linear combination of other angle vectors; this case will hereinafter be described as an implication), the rank of a covariance matrix of a received-signal vector used to estimate the direction of arrival of an echo signal (i.e., the angle formed with the target) is degraded. In this case, a calculation for estimating a direction of arrival according to an inverse matrix or eigenvalue decomposition of the covariance matrix does not hold true, and targets are not detected accurately in some situations.
The degradation of the rank of a covariance matrix may occur due to the position of a target regardless of coherency of received signals. Accordingly, using the spatial smoothing method for eliminating coherency of received signals does not improve the accuracy of estimation of a direction of arrival (this direction is actually an angle formed with a target, and they will not be distinguished from each other in the following descriptions).
As described above, in the prior art, the rank of a covariance matrix of a received-signal vector has been degraded due to, for example, the position of a target, and, even when the target is located within a detection region, the target is not accurately detected in some situations. Accordingly, reliability or practicability of detection and ranging systems has needed to be enhanced.